Adult body mass is a strong correlate of many important life history traits of bees, and thus, has been used as a proxy for these traits in ecological studies. However, body mass is difficult to measure on live specimens in the field, and impossible to measure non-destructively on dry museum specimens. For worker and male bumblebees, we evaluated the linear measures inter-tegular span (IT span) and head width as predictors of fresh and dry weight. For males, linear regressions best explained the relationship between fresh and dry weight and the linear size estimators, while for workers, polynomial regressions best explained this relationship. We found that both IT span and head width are precise estimators of weights of bumblebees. Head width was a slightly better predictor for male weights, while IT span was a slightly better predictor of worker weights. IT span and head width changed little by drying. We discuss patterns of size variation in bumblebees, and highlight possible applications of the method in ecological studies.